Some conventional liquid crystal drivers for digital still cameras (hereinafter, referred to as DSCs) have a resolution converting function for converting a resolution of a YUV format input image data in accordance with a resolution of a display panel for output. In general, converting a resolution in accordance with a resolution of a display panel for output is called scaling.
In scaling, for example, a high-resolution input image data is converted into a low-resolution input image data. The following explains a liquid crystal driver that has the function described above and is used in a conventional compact display, with reference to FIGS. 8 to 10.
(Overview of Conventional Resolution Conversion)
First, the following explains an overview of a conventional resolution conversion, with reference to FIG. 8. FIG. 8 is a schematic diagram illustrating a process for converting input-image image data (hereinafter, referred to as input image data 1) whose pixels are in a stripe arrangement and whose resolution is 720 pixels into image data (hereinafter, referred to as delta arrangement image data 2) whose pixels are in a delta arrangement and whose resolution is 320 pixels. (a) of FIG. 8 schematically illustrates a process for converting the input image data 1 into an odd line data 2a of the delta arrangement image data 2. Meanwhile, (b) of FIG. 8 schematically illustrates a process for converting the input image data 1 into an even line data 2a of the delta arrangement image data 2.
Each of (a) and (b) of FIG. 8 shows a scale 10 or 11 below the input image data 1. The scale 10 is a scale for the input image data 1, that is, a scale having 720 equal divisions of pixels. The scale 11 is a scale for the arrangement image data 2, that is, a scale having 320 equal divisions of pixels. The scale 11 is shown so as to correspond to the scale 10. Regions of the odd line data 2a and the even line data 2b each of which regions is surrounded by a dotted line corresponds to a display area 31 of a display for output.
In the odd line data 2a shown in (a) of FIG. 8, an ellipse 4 indicates pixel data for each one pixel made of sub-pixels of RGB. A scaling ratio used in an example of the present resolution conversion is a ratio of the resolution (720 pixels) of the input image data 1 to the resolution (320 pixels) of the delta arrangement image data 2. Accordingly, the scaling ratio is 2.25. Therefore, a length of data for one pixel in the odd line data 2a corresponds to 2.25 divisions on the scale 10.
Similarly, in the even line data 2b shown in (b) of FIG. 8, an ellipse 6 indicates pixel data for each one pixel made of sub-pixels of RGB. As described above, the scaling ratio is 2.25. Therefore, a length of data for one pixel in the even line data 2b corresponds to 2.25 divisions on the scale 10.
The present resolution conversion is based on linear interpolation. An image data conversion formula used in the liner interpolation requires a predetermined initial value. Initial values used for conversion into the odd line data 2a and the even line data 2b may simply be set as follows. That is, the initial value used for conversion into the even line data 2b in (b) of FIG. 8 may be set to 2.25 and the initial value used for conversion into the odd line data 2a in (a) of FIG. 8 may be set to 2.75 dots which is shifted by 0.5 dot from the initial value used for the conversion into the even line data 2b, in view of the delta arrangement. Here, for convenience of the explanation, the initial value is set to a value obtained by subtracting 1 from each of the above initial values. In other words, the initial value used for conversion into the odd line data 2a in (a) of FIG. 8 is set to 1.75 dots and the initial value used for conversion into the even line data 2b in (b) of FIG. 8 is set to 1.25 dots.
(Conversion of Odd Line)
First, the conversion into the odd line data 2a is explained with reference to (a) of FIG. 8. Conventionally, image data is converted by linear interpolation as follows.
As shown in (a) of FIG. 8, in the conversion into the odd line data 2a, the initial value 1.75 as described above is used. For example, in a case where an R pixel value is converted, a pixel value of r0 of a first pixel provided at a starting position of the odd line data 2a is obtained as follows. Because the initial value used in this case is 1.75, an R pixel corresponding to the initial value in the input image data 1 is a pixel R1. As an arrow 5 indicates, with reference to pixel values of R1 and R2 that is disposed adjacent to R1 on a right side of R1, conversion into r0 is performed. More specifically, a value of r0 is obtained by substituting the pixel values of R1 and R2 into an expression: r0=R1×(1−0.75)+R2×0.75.
Then, a pixel value of r1 of a second pixel in the odd line data 2a is obtained as follows. Because the scaling ratio is 2.25 as described above, a next R pixel to be referred to in the input image data 1 is a pixel corresponding to a position of a value obtained by adding 2.25 to 1.75 dots. That is, as indicated by an arrow 5, with reference to a pixel R4 corresponding to a position of 4 on the scale 10, conversion into r1 in the odd line data 2a is performed. Here, a reference position is 4.00 dot and there is no fractional figure after the decimal point. Accordingly, in this case, with reference to only a pixel value of R4, the conversion is carried out. In other words, r1 is obtained from an expression r1=R4.
Each pixel value is obtained according to the method described above until a value of the last R pixel in the odd line data 2a, that is, r319 is obtained. The same applies to calculation of pixel values of g and b constituting each one pixel of the odd line data 2a. 
(Conversion of Even Line)
Next, the conversion into the even line data 2b is explained with reference to (b) of FIG. 8. Basically, a method for the conversion into the even line data 2b is the same as that into the odd line data 2a as described above.
For example, in a case where an R pixel value is converted, a pixel value of r1 of a first pixel provided at a starting position of the even line data 2b is obtained as follows. Because the initial value used in this case is 1.25, an R pixel corresponding to the initial value in the input image data 1 is a pixel R1. As an arrow 7 indicates, with reference to pixel values of R1 and R2 that is disposed adjacent to R1 on a right side of R1, conversion into r1 is performed. More specifically, a value of r1 is obtained by substituting the pixel values of R1 and R2 into an expression: r1=R1×(1−0.25)+R2×0.25.
Because the scaling ratio is 2.25 as described above, a next R pixel to be referred to in the input image data 1 is a pixel corresponding to a position of a value obtained by adding 2.25 to 1.25 dots. That is, as indicated by an arrow 7, with reference to a pixel R3 corresponding to a position 3.5 on the scale 10, conversion into r2 in the even line data 2b is performed. More specifically, a value r2 is obtained by substituting pixel values of R3 and R4 into an expression: r2=R3×(1−0.5)+R4×0.5.
Each pixel value is obtained according to the method described above until a value of the last R pixel in the even line data 2b, that is, r320 is obtained. The same applies to calculation of pixel values of g and b constituting each one pixel of the even line data 2b. In this way, the resolution converting function of a conventional liquid crystal driver converts the input image data 1 whose pixels are in a stripe arrangement and whose resolution is 720 pixels into the delta arrangement image data 2 whose pixels are in a delta arrangement and whose resolution is 320 pixels.
Next, with reference to FIGS. 9 and 10, the following explains in more detail an arrangement of pixels of image data that is to be converted by the resolution converting function of the conventional liquid crystal driver as described above with reference to FIG. 8. This clarifies a problem of resolution conversion carried out by the conventional liquid crystal driver. First, with reference to FIG. 9, the following explains downsampling.
(Downsampling)
FIG. 9 is a diagram illustrating sampling positions in odd line data and even line data in downsampling in a stripe arrangement, in a case where image data whose resolution is 720 pixels in a stripe arrangement is converted into image data whose resolution is 320 pixels in a delta arrangement. Here, the sampling means to make a reference to a value of pixel data.
As shown in FIG. 9, in stripe arrangement image data 100, pixels S1 to S5 are aligned in the first line (hereinafter, referred to as odd data) and pixels S1′ to S5′ are aligned in the second line (hereinafter, referred to as even data). Scales 101 and 102 shown below the stripe arrangement image data 100 correspond to the first line data and the second line data, respectively. The scales 101 and 102 have numerical values each indicating a starting position of sampling of pixel data in each line.
As shown in the scale 101, an initial value used in sampling of the odd data, that is, the first sampling position is arranged to be 1.75 dots.
A pixel arrangement after the resolution conversion becomes a delta arrangement in which odd data and even data are provided alternately in a vertical direction. Accordingly, regarding pixel units, pixels in the even data is disposed so as to be shifted by 0.5 dot to the left from the pixels in the odd data. Therefore, the initial value used in the sampling of the even data is 1.25 that is obtained by shifting by 0.5 dot the above initial value, in consideration that the arrangement of the pixels after the resolution conversion is a delta arrangement.
Because the resolution conversion is a conversion from the resolution of 720 pixels to the resolution of the resolution of 20 pixels, the scaling ratio is 2.25. Accordingly, on the scale 101, 4 dots is indicated. The “4 dots” is a position of the next sampling which position is obtained by adding 2.25 to the initial value 1.75 in the odd data.
Meanwhile, on the scale 102, 3.5 dots is indicated. The “3.5 dots” is a position of the next sampling which position is obtained by adding 2.25 to the initial value 1.25 in the even data.
According to the above sampling position, a starting position of the first sampling data in the odd data is 1.75 dots. Meanwhile, a starting position of the first sampling data in the even data is 1.25 dots. Further, a starting position of the second sampling data in the odd data is 4 dots. Meanwhile, a starting position of the second sampling data in the even data is 3.5 dots.
Accordingly, with respect to the staring position of the first sampling data in the odd data as a reference position, the starting position of the first sampling data in the even data is shifted by 0.5 dot to the left and the starting position of the second sampling data in the even data is shifted by 1.75 dots to the right.
(Image Data after Conversion)
Next, the following explains an overview of image data after the resolution conversion, with reference to FIG. 10. FIG. 10 is a diagram showing respective positions of RGB pixels in a delta arrangement after conversion of the image data whose resolution is 720 pixels in a stripe arrangement into the image data whose resolution is 320 pixels in the delta arrangement. (a) of FIG. 10 shows a position of G pixel data in the delta arrangement; (b) of FIG. 10 shows a position of B pixel data in the delta arrangement; and (c) of FIG. 10 shows a position of R pixel data in the delta arrangement.
As shown in (a) of FIG. 10, in odd data 110, the first G data is D2, and the second G data is D5. Meanwhile, in even data 111, the first G data is D1′ and the second G data is D4′. Scales 112 and 113 correspond to the odd data 110 and the even data 111, respectively. Each numerical value on the scales 112 and 113 indicates a center position of each pixel data in the odd data 110 or the even data 111. Here, with respect to D2 of the odd data 110 as a reference, D1′ of the even data 111 is shifted by 0.5 pixel to the left and D4′ is shifted by 0.5 pixel to the right.
With reference to (b) of FIG. 10, in the odd data 110, the first B data is D3 and the second B data is D6. Meanwhile, in the even data 111, the first B data is D2′ and the second B data is D5′. Scales 114 and 115 correspond to the odd data 110 and the even data 111, respectively. Each numerical value on the scales 114 and 115 indicates a center position of each pixel data in the odd data 110 or the even data 111. Here, with respect to D3 of the odd data 110 as a reference, D2′ of the even data 111 is shifted by 0.5 pixel to the left and D5′ is shifted by 0.5 pixel to the right.
Further, with reference to (c) of FIG. 10, in the odd data 110, the first R data is D1 and the second R data is D4. Meanwhile; in the even data 111, the first R data is D3′ and the second R data is D6′. Scales 116 and 117 correspond to the odd data 110 and the even data 111, respectively. Each numerical value on the scales 116 and 117 indicates a center position of each pixel data in the odd data 110 or the even data 111. Here, with respect to D1 of the odd data 110 as a reference, D3′ of the even data 111 is shifted by 0.5 pixel to the right and D6′ is shifted by 1.5 pixels to the right.
(Problems in Conventional Resolution Conversion)
As described above, in each of G and B data after the resolution conversion, a shift amount between reference pixel data in the odd data 110 and a pixel in the even data 111 corresponding to the reference pixel data is equal to a shift amount between the reference pixel data in the odd data 110 and a pixel in the even data 111 which pixel is a succeeding pixel of the pixel data in the even data 111 corresponding to the reference pixel data. However, in R data, such shift amounts are not equal. Such uneven shifts cause a display position of each pixel data of a converted image to be misaligned from a display position of each pixel data of an unconverted image. Accordingly, the resolution conversion carried out by the conventional liquid crystal driver may cause a contour section of the converted image to appear jaggy and/or colored (falsely colored). This significantly deteriorates an image quality. Meanwhile, Patent Literature 1 discloses a technique for converting a resolution of an image by a method other than the above-described liner interpolation.
According to the technique of Patent Literature 1, specifically, first, a scaling filter is constructed for resolution adjustment between an input video image of inputted video image signals and an output display device, in the output display device including sub-pixels which output display device has pixels in a delta arrangement. Next, a representing value of sub-pixel values of pixels to be processed by the scaling filter is obtained, and sub-pixel values is obtained in consideration of a difference between the pixels of the input video image. Subsequently, gamma correction suitable for the display device that is to display the sub-pixel values is carried out and the sub-pixel values are displayed by the display device.
The technique of Patent Literature 1 reduces a color fringe that occurs on a boundary of vide images, by a sub-pixel rendering method as described above.
The technique of Patent Literature 1 is a technique of a wide range covering not only scaling but also a process procedure of image processing such as gamma correction. Further, the technique of Patent Literature 1 requires a display that includes a processor that has a fairly high operation processing capability and the process is complicated.
Though an algorithm that is more advantageous for an image quality can be selected for scaling in an environment where a more sophisticated operation processing can be performed, the technique of Patent Literature 1 cannot be applied to a compact display that does not include such a processor as described above.